Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have been long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990's. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a typical terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system 20 may include one or more radiotelephones (terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication between a cell 24 and mobile terminals 22. In this capacity, a base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to a public switched telephone network 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system's service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels. As a practical matter well known to those skilled in the art, radiotelephone communications signals, being modulated waveforms, typically are communicated over predetermined frequency bands in a spectrum of carrier frequencies. In a typical FDMA system, each of these discrete frequency bands serves as a channel over which cellular radiotelephones communicate with a cell, through the base station or satellite serving the cell.
The limitations on the available frequency spectrum present several challenges as the number of subscribers increases. Increasing the number of subscribers in a cellular radiotelephone system requires more efficient utilization of the limited available frequency spectrum in order to provide more total channels while maintaining communications quality. This challenge is heightened because subscribers may not be uniformly distributed among cells in the system. More channels may be needed for particular cells to handle potentially higher local subscriber densities at any given time. For example, a cell in an urban area might conceivably contain hundreds or thousands of subscribers at any one time, easily exhausting the number of channels available in the cell.
For these reasons, conventional cellular systems employ frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Frequency reuse involves allocating frequency bands to each cell, with cells employing the same frequencies geographically separated to allow radiotelephones in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system having only several hundred allocated frequency bands.
Another technique which can further increase channel capacity and spectral efficiency is the use of time division multiple access (TDMA). A TDMA system may be implemented by subdividing the frequency bands employed in conventional FDMA systems into sequential time slots. Communications over a frequency band typically occur on a repetitive TDMA frame structure that includes a plurality of time slots. Examples of systems employing TDMA are those conforming to the dual analog/digital IS-54B standard employed in the United States, in which each of the frequency bands of the traditional analog cellular spectrum are subdivided into 3 time slots, and systems conforming to the GSM standard, which divides each of a plurality of frequency bands into 8 time slots. In these TDMA systems, each user communicates with the base station using bursts of digital data transmitted during the user's assigned time slots.
A channel in a TDMA system typically includes at least one time slot on at least one frequency band. As discussed above, channels are used to communicate voice, data or other information between users, for example, between a radiotelephone and a landline telephone. Channels may be assigned to predetermined slots of predetermined frequency bands, as in the case of dedicated control channels. Included in the typical set of dedicated control channels transmitted in a cell are forward control channels which are used to broadcast control information in a cell of the radiotelephone system to radiotelephones which may seek to access the system. The control information broadcast on a forward control channel may include such things as the cell's identification, associated network identification, system timing information and other information needed to access the radiotelephone system from a radiotelephone.
Channels in a TDMA system may also be dynamically assigned by the system when and where needed. In addition, some systems, such as those conforming to the GSM standard, "frequency hop" traffic channels, i.e., change the frequency band on which a particular traffic channel is transmitted on a frame-by-frame basis. Frequency hopping can reduce the probability of interference events between channels by reducing the likelihood that the same two stations will use the same frequency at the same time. This can help provide for communications quality related to average instead of worst case interference.
Instead of or in addition to FDMA and TDMA techniques, wireless communications systems may employ Code Division Multiple Access (CDMA) or "spread spectrum" techniques. In a CDMA system, a channel is defined by modulating a data-modulated carrier signal by a unique spreading code, i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates. The transmitted signal is demodulated by a receiver unit using the same spreading code using signal correlation techniques. Because the transmitted signal is spread across a wide bandwidth, CDMA communications can be less vulnerable to coherent noise sources which might "jam" other communications signals. The use of the unique spreading code allows several channels to effectively share the same bandwidth.
The quality of service provided by a wireless communications systems such as cellular systems is subject to environmental effects. For example, a cellular radiotelephone call placed under system operating parameters that are designed to produce an acceptable level of communications quality under a set of nominal environmental conditions can be disrupted under "sub-nominal" conditions by fading, shadowing by intervening objects such as hills, and attenuation by distance and by structures such as buildings. Such environmental factors can result in service outages.
An example of such a service disruption occurs when a mobile radiotelephone enters an outage region of a cellular radiotelephone system. Such a region might include a hole in cellular coverage between cells, or an area of degraded reception or transmission within a cell, such as the interior of a building or a tunnel. When the mobile radiotelephone enters such a disadvantaged location, it may be unable to continue a call in progress, to receive notification of an incoming call, or to place an outgoing call.
A wireless communications system can be designed to reduce service disruptions by simply increasing transmit power. Increasing transmit power can be problematic, however, as increasing transmit power can lead to increased inter-channel interference. This can be particularly true in CDMA systems, in which it is generally desirable to balance signal power. In addition, increasing transmit power tends to be impractical for mobile units, as these units typically are power-limited due to size and battery constraints.
In cellular systems, another approach to reducing service outages is to increase the density of cells, i.e., of base stations, so that areas falling between cells are reduced. This approach, however, can lead to increased network complexity, along with increased capital and operational costs associated with the need for additional base stations.
Another approach for providing improved service to subscriber units in disadvantaged locations is to provide a selective high-power paging system that can inform a unit of an incoming call even when the unit is in a disadvantage location. In such a system, a base station sends a paging message to a disadvantaged unit over a specially designated high power channel. The paged unit can then moved to a less disadvantaged location in order to answer the page.
Although this approach can provide high-penetration notifications, this approach generally supports only a small number of users at any given time, as the system is still constrained by the interference concerns described above. Moreover, it may be impractical for a receiving unit to acknowledge such high-power messages while in disadvantaged locations. U.S. Pat. application Ser. No. 08/989,088, assigned to the assignee of the present application, describes a technique for responding to a high-power message which involves sending a simplified acknowledgement comprising a series of binary "1s" over a normal-power channel. However, the information that the mobile unit can transmit according to this technique may be limited.